The residual unsaturated bonds in polymer backbones are susceptible to breakdown when exposed to heat, light or ozone. By hydrogenating these unsaturated bonds, the performance of the material with respect to resistance to heat and ozone can be significantly improved, as well as its durability over long term exposure to aggressive environments.
The process of hydrogenation of unsaturated polymers could be operated in a batch or a continuous manner. It has been known that the batch processes are sometimes expensive, lengthy (a process cycle consists of polymer and catalyst preparation, the setting of reaction conditions, the actual reaction time, followed by the cooling and depressurization of the reactors and the removal of the product) and labour-intensive, and are only suitable for the production of small quantities. Product consistency is difficult to attain. When used for large production volumes, the batch process would require a very large reaction apparatus and very long cycle time. In contrast, when a continuous process is adopted, a large quantity of product can be obtained with consistent qualities and a reactor of a smaller size can be utilized. In addition, the integration of mass balance and heat balance can be realized through a continuous process.
Historically, a continuous process for the hydrogenation of unsaturated polymers generally was carried out in fixed bed reactors, wherein the reactors were packed with various types of heterogeneous catalysts.
U.S. Pat. No. 6,395,841 discloses a continuous process for the hydrogenation of unsaturated aromatic polymers in a fixed bed reactor using a group VIII metal as catalyst. However, a relatively high gas flow rate/polymer solution flow rate ratio (at least 150, vol/vol), was used. The high flow ratio ensured enough hydrogen transfer from hydrogen gas phase to liquid phase. However, the excess of hydrogen gas has to be recycled by a compressor, leading to an increase in operation cost. Furthermore, because of the presence of the packing in the fixed bed reactor, the ratio of the reactor space that can be used for reaction is low.
U.S. Pat. No. 5,378,767 discloses a method of hydrogenating unsaturated polymers with low molecular weight which may contain functional groups such as hydroxyl in a fixed bed wherein the reactor is packed with platinum, palladium or a mixture of the two catalysts supported on an alpha alumina support. This process may not be suited to handle high molecular weight polymers.
U.S. Pat. No. 6,080,372 discloses a continuous stirred tank reactor paired with a bubble column reactor to enhance conversion in a continuous hydrogenation process. Raney nickel catalyst was used to hydrogenate glucose to sorbitol and a reasonably high hydrogenation degree (over 90%) can be achieved. However, the use of a bubble column requires an excess amount of hydrogen to mix gas and liquid and provide sufficient gas-liquid contact.
A multistage agitated contactor (MAC) has been recognized to have many advantages over traditional single stage agitated contactors and bubble column reactors. Only a few applications of MACs as gas liquid contactors have been reported. However, most of the applications are focused on air-water systems at ambient conditions. Regarding the use of a MAC for industrial applications in a continuous manner, very few cases have been reported. U.S. Pat. No. 4,275,012 by Kokubo et al. disclosed its use for the continuous process of refining oils and fats. U.S. Pat. No. 4,370,470 by Vidaurri et al. disclosed its use for the continuous production of arylene sulphide polymer in a MAC. However, only a liquid phase reaction was involved and a gas phase reactant was not involved in the above mentioned processes.
In the prior art, it can be seen that only fixed bed reactors have been employed for the continuous process for hydrogenation of unsaturated polymers, particularly, using heterogeneous catalysts. However, some valuable polymers are obtained by using more efficient and highly selective homogeneous catalysts. For example, Rempel, G. L., 2000, Catalytic Hydrogenation of Nitrile Butadiene Rubber, Polymer Preprints, 41(2), 1507 reported several effective catalysts for selective hydrogenation of nitrile butadiene rubber, including rhodium, ruthenium and osmium catalysts. However, these homogeneous catalyst systems are conducted in batch processes.
When fixed bed reactors are applied to perform the continuous process, it is not economical due to the low applicable reactor volume ratio and high pressure drop. If a single continuous stirred tank reactor (CSTR) is considered, an extremely large reactor is needed for a long reaction time since a high hydrogenation degree (95%) is usually required for the production of the final polymer. A continuous process for polymer hydrogenation has some special requirements such as instantaneous mixing of the catalyst at the inlet, exothermic peak mitigation and backflow prevention between the two stages; therefore, the existing MACs mentioned above are not applicable for polymer hydrogenation.